Thank you to 23andMe for supporting PBS digital studios Simone: Are we ready to try this? Diana: I think we’re ready to try it Simone: What do you think is going to happen? Diana: I honestly haven’t thought about it.

Do you know what’s going to happen? Oh no! *screams Hey, I’m Diana you’re watching Physics Girl and you may have seen this riddle before it went pretty viral in 2016 It’s a simple riddle it just asks which of these containers will fill up first I happen to be in San Francisco and so I thought who’s my favourite person with tools who would spend all afternoon with me building something useless We went to the hardware store and bought some containers of screws Just to empty the containers of the screws and use those as our buckets So then after this hardware store ordeal We screwed our very accurate model of the riddle to some plywood.

I provided the hot glue gun. But first: predictions like good little scientists. Diana: And which one is gonna fill first? Simone: As soon as it comes close to this pipe It’s gonna start trickling down here same thing happens here it’s gonna go along this pipe so the water level here is gonna increase It’s going to start trickling over here it’s gonna rise there and then these ones are gonna rise to the top together that’s my idea Diana: I think that you’re right but I wonder if these ones will ever fill up? Simone: No ’cause these ones are gonna top out before these ones do Diana: Yeah they’re gonna spill.

I mean, I guess there has to be like some restraints There has to be some like a premise There are so many assumptions you need to make about this riddle if you actually want to answer it with physics I asked you guys on twitter what missing assumptions made you mad about this riddle and you gave me some pretty good ones Which way is gravity pointing? We’re gonna assume down What does filling mean? Does it mean like ‘oh this glass is full’ or does it mean like FILL IT TO THE BRIM BARB! We’ll assume filled to the brim. How fast is the water coming in through the top compared to how fast it’s moving through the rest of the pipes? We’re gonna assume it’s a relatively slow trickle so those were our important assumptions going into this If I have one criticism of viral memes it’s that they don’t specify their physical assumptions in enough detail Diana: Let’s do it Simone: Yeah I am like really excited. Simone: This is the smallest trickle. Diana: It is a pretty small trickle. We sat through too many minutes of this So you don’t have to Diana: This is going to take a really long time Simone: Yeah it is OOHHH! Yeah, yeah, yeah! Diana: Look, look. So number one has stopped filling Simone: It’s in number three Diana: NOOO!! Simone: I told you! We had a leak *sniffs So we took the opportunity to fix the little error in our design. The inlet from three to four went too high before it went into four so we had to move four down but then four was now lower than three Urgh! What are you gonna do? Then we came back and we used strawberry lemonade because it was easier to see. Diana: Oh are you gonna siphon it? Simone: Yeah is that what it’s called? Diana: Yeah Simone: I only have practical skills no theoretical Okay, so you can see pretty quickly that it’s not gonna be one or two because as soon as they fill up to their outlets, they start to pour out into the next bucket so the question is now: Three or Four? Simone: Ahhh nail-biter! Simone: I think your arm might be blocking it Diana: What is happening? Ahh! And all was well until. . . Oh! Oh no!! *screams Strawberry lemonade everywhere! Oh my god Where did it even go? We’re good, we’re good.

That is your definition of we’re good? You’re friend is like dying over here and you’re like The strawberry lemonade is fine. You physicist you only care about your experiments It’s true. Simone: I’m impatient. Diana: And now this one’s gonna spill first Simone: Well, that’s exciting Diana: Oh yeah, we’ve got this one spilling But this is lower than that because there were imperfections so Yeah if they would have been the exact same level they would have started spilling at the same time and these two would never fill up So for us, four spilled first but we had moved it down below three because our inlet was too high Remember? So then the question becomes in an ideal situation will three fill first or will four fill first? Here’s the thing: the physics that this riddle is trying to get at although who can really know with viral riddle memes but I think that it’s trying to get at this contraption Diana: Who would ask you to do something like this? Kyle: On a monday night after I’ve worked all day? Somebody who knows I can’t resist a CADD Thanks Kyle. So this is a model of Pascal’s Tubes.

When you fill from any one of the containers they will all stay at the same level and this always holds true because of Pascal’s Law Pascal’s Law says that a pressure change like for example adding the weight of more fluid to one part of the container will be transmitted without loss through the entire fluid to all parts of the container equally because it’s an incompressible fluid you can’t change the volume by changing the pressure and water is approximately incompressible. Pascal’s Law is shown mathematically like this So, that suggests that once three starts spilling into four three and four should start filling at the exact same rate if the flow is coming into the top infinitesimally slowly But is that what really happens? That’s where it’s like impossible to say it depends on how perfectly you cut your materials. Simone: I might have gone a little bit wild with the jigsaw over there. Like, give a girl a break It depends on how fast the fluid is flowing in. So in the end the result is for all intents and purposes three and four fill about at the same time to the brim. Which one spills first? Who knows we don’t know! The riddle is a trick question because it leaves too many things unknown AHH! Just because you sort of know what happens with this riddle and will promptly forget next week doesn’t mean that you’re ready for this riddle I’ll put the answer in the description.

Did you guess correctly? I don’t know maybe you might not have seen this block in it. It’s infuriating! And then of course there’s this riddle. Oh, physics If you made it this far with me in the video thank you. Hey, Subscribe to Simone Giertz she’s got some great machines and some interesting robots *spits Simone: This not being one if them Going into the third season of videos with PBS digital studios I’ll be posting videos every other Wednesday Put it in your calendar And for this year you can expect some more guests you may have noticed that I have a riddle series with guests Let me know if there’s anyone you would maybe want to see me ask physics riddles to or if there’s any riddles you would want me to ask that person hit subscribe and the notification bell if you want to know when those videos come out every two weeks and I have to thank the sponsor of this episode 23andMe for supporting me and for supporting PBS digital studios the name 23andMe comes from the fact that human DNA is organized into 23 pairs of chromosomes 23andMe’s personal genetic service was created to help people understand the DNA that they carry Think about this: Your 23andMe results might help you become closer to your family as you learn more about the genes that you share Imagine this: I’m sitting at the thanksgiving table with my family and comparing who has the most Neanderthal DNA You can really do that Now through turkey day, 23andMe has a special thanksgiving family offer so go to to check it out and get kits for your family today make sure to use our url and if you give it a try let me know. Happy physics-ing



Please enter your comment!
Please enter your name here